1. Field of the Invention
The invention relates to optical scanning systems, and more particularly, to apparatus for converting an optical image to electrical signals.
2. Description of the Prior Art
Prior scanners utilizing self-scanning linear arrays exist for scanning documents and converting the images scanned into electronic video-type signals. These types of scanners have the disadvantage that the scanner must be manually moved across the document, or the document must be moved relative to the scanner in order to scan the image.
Apparatus generally referred to as electronic cameras have been developed in order to provide a system capable of scanning a document while the scanner or camera is held stationary.
The prior art has developed along two separate paths. One type of system utilizes a matrix of solid-state, light-sensitive arrays at the image plane, wherein both the array and the lens are held stationary. The other type of system employs a linear array comprised of a single row of light-sensitive devices. A two dimensional image scan is achieved by electrically scanning in a first direction and mechanically scanning in a second direction perpendicular to the first direction.
An example of the first type of syste is shown in the Walker U.S. Pat. No. 4,025,910 which discloses a stationary array of storage elements located at the image plane of a camera. The array is an XY matrix of light-sensitive storage cells which are held stationary at the image plane i order to convert the image focused thereon into electrical signals. This type of system has the disadvantage of low resolution because of the low density of storage cells which can be fabricated in the array.
Examples of the second type of system are shown in the Berler U.S. Pat. No. 3,600,593 and Lianza U.S. Pat. No. 4,256,969 which disclose scanners in which a single row of photocells at the image plane convert an image focused thereon by a lens into electrical signals. A two-dimensional line scan of the image is achieved by electrical scanning in one direction and mechanical scanning in a perpendicular direction.
Neither the Berler patent or the Lianza patent disclose means for synchronizing the sequential array scans with the mechanical position intervals. This could result in loss of sampled data due to the differences between the actual position of mechanical parts of the apparatus and the clock signals that trigger the array scan.
A further example of the second type of system is described in an article by E. R. Williams, entitled Two-dimensional Image Sensor Using One-dimensional Sensor Array, published in the IBM Technical Disclosure Bulletin, Vol. 21, No. 2, July 1978. This article shows a scanner in which a one-dimensional array is mounted on an oscillating member. The frequency of oscillation and the sampling of the array are determined by a common clock. There are several disadvantages to such a device.
One disadvantage of the Williams device is that the sensor array follows a curved path and hence its movement does not match the image focal plane of a standard 35 mm camera lens. The image will be in focus only for very short excursions of the oscillating member, unless a specially designed negative lens is used to match the negative cylindrical curvature followed by the array. Thus the device cannot use standard, interchangeable lenses.
Another disadvantage of the Williams device is that, like Lianza and Berler discussed above, no means are provided for synchronizing the sequential array scans with the mechanical position intervals of the array as it moves in its curved path. This results in loss of sampled data at the far extremes of travel of the oscillating member, due to the difference between the actual mechanical position of the array and the clock signals that trigger the array scan.
A further disadvantage of the Williams device is that because the tuning fork oscillates, the speed of the array cannot be varied across the image plane. Because mechanical-to-electrical synchronization cannot be maintained, the array in Williams cannot be driven incrementally across the image plane to accommodate a wide variety of image capturing requirements.